The present invention relates generally to multiple description transform coding (MDTC) of data, speech, audio, images, video and other types of signals for transmission over a network or other type of communication medium.
Multiple description transform coding (MDTC) is a type of joint source-channel coding (JSC) designed for transmission channels which are subject to failure or xe2x80x9cerasure.xe2x80x9d The objective of MDTC is to ensure that a decoder which receives an arbitrary subset of the channels can produce a useful reconstruction of the original signal. A distinguishing characteristic of MDTC is the introduction of correlation between transmitted coefficients in a known, controlled manner so that lost coefficients can be statistically estimated from received coefficients. This correlation is used at the decoder at the coefficient level, as opposed to the bit level, so it is fundamentally different than techniques that use information about the transmitted data to produce likelihood information for the channel decoder. The latter is a common element in other types of JSC coding systems, as shown, for example, in P. G. Sherwood and K. Zeger, xe2x80x9cError Protection of Wavelet Coded Images Using Residual Source Redundancy,xe2x80x9d Proc. of the 31st Asilomar Conference on Signals, Systems and Computers, November 1997.
A known MDTC technique for coding pairs of independent Gaussian random variables is described in M. T. Orchard et al., xe2x80x9cRedundancy Rate-Distortion Analysis of Multiple Description Coding Using Pairwise Correlating Transforms,xe2x80x9d Proc. IEEE Int. Conf. Image Proc., Santa Barbara, Calif., October 1997. This MDTC technique provides optimal 2xc3x972 transforms for coding pairs of signals for transmission over two channels. However, this technique as well as other conventional techniques fail to provide optimal generalized nxc3x97m transforms for coding any n signal components for transmission over any m channels. Moreover, the optimality of the 2xc3x972 transforms in the M.T. Orchard et al. reference requires that the channel failures be independent and have equal probabilities. The conventional techniques thus generally do not provide optimal transforms for applications in which, for example, channel failures either are dependent or have unequal probabilities, or both. This inability of conventional techniques to provide suitable transforms for arbitrary dimensions and different types of channel failure probabilities unduly restricts the flexibility of MDTC, thereby preventing its effective implementation in many important applications.
The invention provides MDTC techniques which can be used to implement optimal or near-optimal nxc3x97m transforms for coding any number n of signal components for transmission over any number m of channels. A multiple description (MD) joint source-channel (JSC) encoder in accordance with an illustrative embodiment of the invention encodes n components of a signal for transmission over in channels of a communication medium, in applications in which at least one of n and m may be greater than two, and in which the failure probabilities of the m channels may be non-independent and non-equivalent. An nxc3x97m transform implemented by the MD JSC encoder may be in the form of a cascade structure of several transforms each having dimension less than nxc3x97m. An exemplary transform in accordance with the invention may include an additional degree of freedom not found in conventional MDTC transforms. This additional degree of freedom provides considerable improvement in design flexibility, and may be used, for example, to partition a total available rate among the m channels such that each channel has substantially the same rate.
In accordance with another aspect of the invention, an MD JSC encoder may include a series combination of N xe2x80x9cmacroxe2x80x9d MD encoders followed by an entropy coder, and each of the N macro MD encoders includes a parallel arrangement of M xe2x80x9cmicroxe2x80x9d MD encoders. Each of the M micro MD encoders implements one of: (i) a quantizer block followed by a transform block, (ii) a transform block followed by a quantizer block, (iii) a quantizer block with no transform block, and (iv) an identity function. This general MD JSC encoder structure allows the encoder to implement any desired nxc3x97m transform while also minimizing design complexity.
The MDTC techniques of the invention do not require independent or equivalent channel failure probabilities. As a result, the invention allows MDTC to be implemented effectively in a much wider range of applications than has heretofore been possible using conventional techniques. The MDTC techniques of the invention are suitable for use in conjunction with signal transmission over many different types of channels, including lossy packet networks such as the Internet as well as broadband ATM networks, and may be used with data, speech, audio, images, video and other types of signals.